1. Technical Field
The present invention is concerned with wireless transmission, and methods for choosing one of a plurality of candidate stations for participating in such transmission. One particular context of interest is the selection of relay stations in a wireless local area network environment (or, for short, wireless LAN or simply WLAN), especially wireless LANs in accordance with the IEEE 802.11 standard. However the invention may be used in other contexts, too.
2. Related Art
There have been several proposals for implementing multi-hop functionality in an 802.11 WLAN. Most of them perform relaying at the MAC layer. This is essentially because MAC relaying does not include queuing delays and does not affect the relay node transmission opportunity compared with route forwarding. Zhu and Cao proposed two relay based protocols namely, Relay-Enabled PCF and Relay-Enabled DCF. The first of these protocols is described in H. Zhu and G. Cao, “On improving the performance of IEEE 802.11 with Relay-Enabled PCF,” ACM/Kluwer Mobile Networking and Applications (MONET), 2004. There, nodes maintain transmit tables that contain notification of channel transmission rate changes by access point (AP). This notification information is frequently piggybacked with packets to cope with channel modification and nodes' mobility. Maintaining tables and frequent notification result in excessive overhead. Relay-Enabled DCF is described in H. Zhu and G. Cao, “rDCF: A Relay-enabled Medium Access Control Protocol for Wireless Ad Hoc Networks,” IEEE Trans. on Mobile Computing, 2005. In this proposal, there is the use of rate adaptation that is similar to rPCF. This protocol selects a good relay for transmission but it may not be the best one. Both protocols are proven to achieve good performance results. In the same distributed environment as rDCF, Bletsas et al propose a new method to assess the quality of the channel (see A. Bletsas, A. Lippman and D. P. Reed, “A Simple Distributed Method on Relay Selection in Cooperative Diversity Wireless Networks, based on Reciprocity and Channel Measurements,” IEEE 61st Semiannual VTC, May 30-Jun. 1, 2005, Stockholm Sweden). This method is simple and practical because it gives instantaneous feedback of the channel condition. It assumes that the transmission channel in both directions (sending and receiving) is the same, and the nodes have the same power in order to select the best of them comparing only the quality of the channel, i.e. it does not take into account that there is a variety of mobile stations with different power settings in a WLAN.